CORRELATION DETECTOR AND COMMUNICATION APPARATUS

Abstract: A correlation detector for CDMA communication receivers, which can establish synchronization at a high speed, can extract mutually correlated components, and can perform highly accurate tracking. The received signal (21) is fed to a matched filter (43) when the initial synchronization is established. When a synchronization judging circuit (45) judges that the filter (43) detects synchronization, the switching circuit (42) feeds the signal (21) to multipliers (47, 48), and resets a VCCG (29) and a spread-spectrum code replica generator (30). After the synchronization is established, the spread-spectrum signal is detected quasi-synchronously, the received signal quasi-synchronously detected is multiplied by replical codes having advanced and lagged phases, and correlation detection signals are extracted from the multiplication results. The square components are generated, added together in opposite phases, and averaged in the time direction. The received spread-spectrum signal is multiplied by the codes whose phases are synchronous, and the results are integrated over chips. The signal, after the averaging processing, is reversely modulated by use of the identification signal of the received data obtained by compensation for the phase error of the received signal in order to generate a phase error signal. The replica code generator is driven by a clock the phase of which is controlled by the phase error signal.

Description: TECHNICAL FIELD The present invention relates to a correlation detector of a radio receiver in a CDMA (Code Division Multiple Access) system which carries out multiple access by using a spread spectrum in mobile communications. In particular, the present invention relates to a CDMA synchronizing circuit that synchronizes a spreading code for despreading the received signal to a spreading code in a received signal in CDMA communications. BACKGROUND ART CDMA communications perform multiple access propagation by spreading information into wideband signals using spreading codes with rates higher than the rate of the information, and are roughly divided into direct sequence (DS) systems that spread modulated signals by high rate spreading codes, and frequency hopping (FH) systems. The FH system resolves each symbol into smaller elements called chips, and translates the chips into signals with different center frequency at a high speed. Since the implementation of the FH system is difficult, the DS system is generally used. The DS system recovers the original narrowband signal by despreading the wideband received input signal at a receiving end, followed by demodulation. In the despreading process, correlation detection is performed between the spreading code included in the received signal and a spreading code generated at the receiving end. Thus, the receiver for receiving the spread signal in the DS system is usually provided with a replica (reference PN sequence) of the PN sequence (received PN sequence) in the received signal, and establishes synchronization between the reference PN sequence and the received PN sequence. Fig. 1 shows a conventional synchronization circuit using a matched filter. The received signal applied to an input terminal 10 is supplied to a memory circuit 11 with taps. The number of taps of the tapped memory circuit 11 is the same as the number of chips in a spreading code interval (that is, a processing gain PG). The outputs of the taps of the memory circuit 11 are multiplied by the reference spreading code stored in a tap coefficient circuit 13 by multipliers 12. The resultant products are summed by an integrator 14, which outputs the sum from its output terminal 16 as a correlation value 15. Using the matched filter makes it possible to quickly establish the synchronization because the peaks of the correlation value appear at the same interval as that of the spreading code. However, since the capacity of the tapped memory circuit 11 and the number of the multipliers 12 increase in proportion to the processing gain, the power consumption of the receiver will increase with the interval of the spreading code. Therefore, the conventional synchronizing circuit is not appropriate for portable devices or mobile devices. Using a sliding correlation detector as shown in Fig. 2 makes possible power saving and downsizing of the circuit. In Fig. 2, a received signal 21 inputted to the input terminal 10 is multiplied by a spreading code, which is generated by a spreading code replica generator 30, by a multiplier 22 to obtain the correlation between the two. The resultant product is passed through a bandpass filter (BPF) 23, followed by peak power detection by a square-law detector 24. The detected power is integrated over a fixed time (normally, ± one chip interval) by an integral-dump circuit 25. The integrated result is compared with a threshold value by a threshold value decision circuit 26 which decides that initial acquisition has been completed if the integrated result exceeds the threshold value, and proceeds to the next step (tracking mode). If the integrated result is less than the threshold value, the decision circuit 26 supplies a control voltage 28 to a voltage controlled clock generator (VCCG) 29 which slides the phase of the replica so that the phase of the spreading code generated by the spreading code replica generator 30 is shifted by 1/N chip interval (N is a natural number equal to or greater than one). The initial acquisition has been completed by repeating the processing until the synchronized point is found. According to this method, it is necessary to integrate the spreading replica over the fixed time every time the replica is shifted by 1/N chip interval, and to detect the synchronized point in the interval of the spreading code by comparing the integrated result. This will lengthen the acquisition time, and hence, it is not appropriate for a system which requires a quick acquisition. In addition, the conventional correlation detector presents another problem in that it provides a rather large error in maintaining (tracking) the synchronization. Fig. 3 is a block diagram showing a conventional DLL (Delay Locked Loop) correlation detector 44. In Fig. 3, the same functional blocks are designated by the same numerals as in Fig. 2. The reference numeral 10 designates a spreaded signal input terminal, 102 designates a decided data output terminal, 111 denotes a multiplier, and 510 designates a delay circuit. The correlation detector 44 calculates correlations between the input modulated signal and code sequences formed by advancing and retarding the chip phase of the replica by 1/N, respectively. The correlated signals are passed through bandpass filters (BPFs) 53 and 54 which eliminate unnecessary high frequency components, and are detected by square-law detectors 55 and 56. The squared amplitude components are summed by an adder 57 in the opposite phase, so that an error signal voltage is obtained which indicates an amount of a phase difference. The error signal voltage is passed through a loop filter 58, and is fed back to a VCCG 29 to correct the phase of the replica code sequence. The phase advance (or retardation) time δ ranges from 0 to Tc, where Tc is the chip interval. Applying the CDMA system to cellular communications requires high accuracy transmission power control that keeps constant base station's received levels of signals sent from all the mobile stations. The CDMA system can increase the capacity in terms of the number of subscribers per frequency band as compared with the FDMA system or the TDMA system. This is because conventional systems which employ frequency orthogonality cannot utilize the same carrier frequencies in the contiguous cells, and even space diversity systems cannot reuse the same frequencies within four cells. In contrast with this, the CDMA system makes it possible to reuse the same carrier frequency in the contiguous cells because the signals of the other communicators are regarded as white noise. Accordingly, the CDMA system can increase the capacity in terms of the number of subscribers as compared with the FDMA system or the TDMA system. If the processing gain is pg, the number of spreading code sequences that completely orthogonalize with each other is pg. This number of the code sequences, however, will be insufficient when information data is spread by using only code sequences of one symbol interval long. To overcome this problem, the number of the spreading codes is increased almost infinitely by superimposing long code sequences of a very long interval over short code sequences of one symbol interval. Unlike M sequences that have definite autocorrelation characteristics, the autocorrelation of Gold sequences and that of the sequences obtained by superimposing very long code sequences over the Gold sequences will have undesired peaks of considerable amplitudes in addition to the normal correlation peak in one symbol interval. As a result, when the received signal level is low, a lock may be lost in the conventional delay-locked loop using one chip interval lock. Let us formulate the operation principle of the delay-locked loop of Fig. 3. First, the input signal is expressed by the following equation. where S is average signal power, c(t-τ t ) is a received spreading code including a propagation delay, m(t-τ t ) represents data modulation including the propagation delay, ω 0 is the angular frequency of a carrier, and (t) =  0 + Ω 0t is an unknown carrier phase which is represented as the sum of a constant term and a term proportional to the Doppler frequency. The power spectrum density of n(t) is N 0/2 . δω 0 is an angular frequency error between the center frequency of a modulation signal and a local oscillation frequency. In addition, the bandpassed expression of input thermal noise ni(t) is given by n i (t) = 2 {N c (t) • cos[Δω 0 t + Δ(t)] - N s (t) • sin[Δω 0 t + Δ(t)]} where N C (t) and N S (t) are assumed to be approximately and statistically independent and steady. The spreading replica sequence of the advanced phase and that of the retarded phase can be expressed as follows: where τ and t is a propagation delay estimated by the DLL at the receiving side. The crosscorrelation output of the phase detector is expressed as where K m is the gain of the phase detector which is assumed to be equal in both branches, and × represents the average of a set. Figs. 4A - 4B illustrate the autocorrelation outputs in terms of the received chip phase error. Here, is a normalized propagation delay error. H(s) is a lowpass expression of a transfer function H(s) of the bandpass filter, and is a process of a PN sequence. The output of the square-law detector can be expressed as follows using R PN± (x) which is a function obtained by shifting the autocorrelation function of PN by a time period of +x. where Here, H/(p)×(t) expresses an output response of the BPF to x(t). If the bandwidth BL is sufficiently smaller than the chip rate, the effect of the auto-noise caused by the PN sequence on the loop is negligible in the first-order approximation. Neglecting the auto-noise and the secondary harmonic caused by the square-law detection, the input to the loop filter can be expressed by the following equation. where According to the foregoing, a normalized delay estimate of the output of the spreading code replica generator is expressed by the following equation using e(t). τ t T c = K VCC F(P) P e(t) where F(s) is the transfer function of the loop filter, and K VCC is the gain of a voltage controller in the VCCG which drives the PN sequence generator. Placing K = Km 2 K VCC , K represents the loop gain. Substituting equation (7) into (8), Thus, estimated error ε t is expressed as Resolving the first term in the blanket of the above equation into an average value term and modulated auto-noise term gives where < > expresses an average in time, and where Sm(f) is a power spectrum density of the data modulation. The M 2 term is the integral of the data modulation power spectrum density over the passband of the filter, and indicates the data modulation power in the passband. Since the bandwidth of the loop is much smaller than the data symbol rate, the auto-noise associated with the second term of equation (11) is negligible. From equation (10), the following equation is obtained. where a dot placed over characters represents a time differential, and η is given by η ≡ 4(N-1) N Briefly, the average of squared tracking jitter due to noise component is expressed as follows: where B L is an equivalent noise bandwidth of the LPF, and Ne(t) is expressed as where f(ε t ) represents a square-law detection curve. Since the conventional DLL uses the square-law detector as shown in equation (15), the noise component is also squared. This will increase the tracking jitter as shown in equation (14). A paper entitled "A Modified PN Code Tracking Loop" by Yost et al (IEEE Transactions on Communications, Vol. Com-30, No. 5, May 1982) describes a non coherent PN code tracking loop where, instead of using the sum channel as the reference, the on time or data channel is utilised. US patent 4468784 describes a mixer driver circuit in a code correlator for a spread spectrum communications system. The mixer driver circuit simultaneously multiplies an input RF signal with two phase offset digitally encoded bit streams. US patent 4841544 describes a spread spectrum receiver which converts a received signal into an IF signal centred about a frequency corresponding to the code fundamental frequency offset by a Doppler shift. The IF signal is then digitised and sampled at a local clock frequency to develop N-bit digital signals. In phase and quadrature component signals are then derived from the digitised IF signals. The tracker uses the component signals to track the Doppler shift, to maintain the local sampling frequency and to maintain the local code generator in synchrony with the code of the received signal. DISCLOSURE OF THE INVENTION Therefore, an object of the present invention is to provide a low power consumption CDMA synchronising circuit capable of high speed synchronisation. Another object of the present invention is to provide a correlation detector that can perform high accuracy tracking capable of eliminating the square loss resulting from the emphasis of the noise component by the square law detector, which differs from the conventional code tracking circuit. A first aspect of the invention provides a synchronising apparatus including a correlation detector for maintaining synchronisation between a received signal and a despreading code by using a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said correlation detector comprising: a spreading code replica generating means for generating a phase advanced replica of a CDMA spreading code with an advance phase and a phase retarded replica of the CDMA spreading code with a retarded phase with respect to the received signal; first multiplying means for multiplying said received signal by said phase advanced replica of the CDMA spreading code so as to output a first correlation detection signal; and second multiplying means for multiplying said received signal by said phase retarded replica of the CDMA spreading code so as to output a second correlation detection signal; the apparatus being characterised in that it comprises: carrier frequency error compensating means for compensating for a carrier frequency error associated with said first and second correlation detection signals; and adding means for summing in opposite phase said first and second correlation detection signals, which have been compensated by said carrier frequency error compensating means; an averaging circuit for averaging an output signal of said adding means along a time axis; and wherein said spreading code replica generating means is further operable to generate an in phase replica of said CDMA spreading code with the same phase, and wherein the correlation detector further comprises: third multiplying means for multiplying said received signal by said in-phase replica of the CDMA spreading code from said spreading code replica generating means; integrating circuit means for integrating an output signal of said third multiplier means; automatic frequency control circuit means for detecting the carrier frequency error based on an output of said integrating circuit means, to output it to said carrier frequency error compensator; a demodulator for generating a decided signal of received data based on an output signal of said automatic frequency control circuit means; an inverse modulation circuit for inversely modulating an output signal of said averaging circuit by the decided signal of the received data; and clock generating circuits for generating a clock signal whose phase is controlled by an output from said inverse modulation circuit to control said spreading code replica generator. A second aspect of the invention provides a synchronising method using a correlation detector for tracking between a received signal and a despreading code by the use of a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said method comprising the steps of: generating an in-phase replica of a CDMA spreading code with the same phase, a phase advanced replica of the CDMA spreading code with an advance phase and a phase retarded replica of the CDMA spreading code with a retarded phase with respect to the received signal; firstly multiplying said received signal by said phase advanced replica of the CDMA spreading code so as to output a first correlation detection signal; secondly multiplying said received signal by said phase retarded replica of the CDMA spreading code so as to output a second correlation detection signal; compensating for a carrier frequency error associated with said first and second correlation detection signals; summing in opposite phase said first and second correlation detection signals, which have been compensated at said carrier frequency error compensating step; averaging an output at said summing step along a time axis; third multiplying said received signal by said in-phase replica of the CDMA spreading code; integrating an output at said third multiplying step; detecting the carrier frequency error based on an output at said integrating step so as to use it at said carrier frequency error compensating step; generating a decided signal of received data based on an output at said integrating step; inversely modulating an output at said averaging step by the decided signal of the received data; and controlling a phase of a replica code to be output based on the output at said inversely modulating step. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a synchronizing circuit or an initial acquisition circuit of a conventional matched filter; Fig. 2 is a block diagram showing a conventional sliding correlator; Fig. 3 is a block diagram showing a conventional DLL. Figs. 4A-4C are diagrams illustrating crosscorrelation output signals in terms of a phase error of received signal; Fig. 5 is a block diagram showing a basic arrangement of a first embodiment of a correlation detector in accordance with the present invention; Fig. 6 is a block diagram showing another basic arrangement of the first embodiment of a correlation detector in accordance with the present invention; Fig. 7 is a block diagram showing details of hardware of the correlation detector in accordance with the present invention; and Fig. 8 is a block diagram showing another embodiment of a correlation detector in accordance with the present invention. BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the invention will now be described with reference to the accompanying drawings. EMBODIMENT 1 Fig. 5 shows an embodiment of the present invention. A received signal 21 applied to the input terminal 10 is selectively supplied through a switching circuit 42 to an initial acquisition circuit 43 consisting of a matched filter or to a correlation detector 44. The initial acquisition circuit 43 has an arrangement similar to that of Fig. 1. The correlation detector 44 has a function similar to the correlation detectors of Figs. 2 and 3. When the initial acquisition has not yet been completed, the received signal is supplied to the initial acquisition circuit (matched filter) 43 in accordance with a switching signal 46 from an acquisition decision circuit 45, so that the correlation detection is performed. The correlation value detected by the matched filter 43 is compared with a threshold value in the acquisition decision circuit 45. If the correlation value is greater than or equal to the threshold value, the acquisition decision circuit 45 decides that the initial acquisition has been completed, and changes the switching circuit 42 using the switching signal 46. Thus, the received signal is inputted to multipliers 47 and 48 in the correlation detector 44. The acquisition decision circuit 45 provides an initial reset signal 49 to a VCCG 29 and a spreading code replica generator 30. The received signal 21 after the initial acquisition is multiplied by spreading codes 51 and 52 by the multipliers 47 and 48, the spreading codes being generated by the spreading code replica generator 30, and having phases shifted forward and backward in time by an amount T (less than one chip interval). The two products are passed through bandpass filters (BPF) 53 and 54, and are square-law detected by square-law detectors 55 and 56, in which correlation values are detected. The correlation values are summed in the opposite phase by an adder 57. The sum is passed through a loop filter 58, and becomes a control voltage of the VCCG 29. The clock signal generated by the VCCG 29 regulates the phase of the spreading code replica generator 30, tracks the synchronous point, and maintains the synchronization. Fig. 6 illustrates an example, in which the spreading code replica generator 30I is synchronized with the outputs of the quadrature detector 62 which detects the received signal 21 applied to the input terminal 10. In this figure, portions corresponding to those of Fig. 5 are designated by the same reference numerals. Suffixes I and Q are attached to the same numerals as in Fig. 5 to represent processing circuits of the two detected outputs I and Q of the quadrature detector 62. The detected outputs I and Q are passed through lowpass filters 63 and 64, converted into digital signals by A/D converters 65 and 66, and supplied to switching circuits 42I and 42Q. The outputs of the initial acquisition circuits 43I and 43Q are square-law detected by square-law detectors 71 and 72, summed by an adder 73, and supplied to the acquisition decision circuit 45 which decides whether the initial acquisition is established or not. A spreading code replica generator 30I in the correlation detector 44 generates an advanced spreading code 51I with an advanced phase and a retarded spreading code 52I with a retarded phase. Likewise, a spreading code replica generator 30Q generates an advanced spreading code 51Q with the advanced phase and a retarded spreading code 52Q with the retarded phase. The detected output component I outputted from the switching circuit 42I is multiplied by the advanced spreading code 51I and the retarded spreading code 52I by multipliers 47I and 48I. Likewise, the detected output component Q outputted from the switching circuit 42Q is multiplied by the advanced spreading code 51Q and the retarded spreading code 52Q by multipliers 47Q and 48Q. The detected correlation values with the advanced spreading codes 51I and 51Q, which are outputted from the multipliers 47I and 47Q, are passed through bandpass filters (BPFs) 53I and 53Q, square-law detected by square-law detectors 55I and 55Q, and are summed by an adder 67. Similarly, the detected correlation values with the retarded spreading codes 52I and 52Q, which are outputted from the multipliers 48I and 48Q, are passed through bandpass filters (BPFs) 54I and 54Q, square-law detected by square-law detectors 56I and 56Q, and are summed by an adder 68. The outputs of the adders 67 and 68 are summed in the opposite phase by the adder 57. These operations are similar to those of Fig. 5. According to the first embodiment, the synchronization process is separated into an initial acquisition process, and a tracking process using the correlation detector. The input PN sequence is acquired so that the phase difference between the input PN sequence and the reference PN sequence is settled within a range sufficiently smaller than ± one chip interval during the initial acquisition because the autocorrelation of the PN sequence is established only within a range of ± one chip. The tracking processing holds the phase difference between the input PN sequence and the reference PN sequence within the range. EMBODIMENT 2 Fig. 7 is a block diagram of a correlation detector of a second embodiment. In Fig. 7, the same functional blocks are designated by the same reference numerals as in Figs. 1 - 6. As shown in Fig. 7, a signal applied to the input terminal 10 is quasi-coherent detected by a detector 104 using a local signal generated by a local oscillator 103. The local signal has a fixed frequency substantially equal to the center frequency of the modulated signal. The correlation detector includes multipliers 47 and 48 for detecting correlations between the received spreading code and replicas of the spreading code; bandpass filters 83 and 84 for extracting only correlation detection signals from the products; a carrier frequency error compensator 208 for compensating the filtered output signals with a carrier frequency error signal detected by an automatic frequency control circuit; an adder 57 for summing in the opposite phase the correlation detection signal associated with an advanced phase replica and the correlation detection signal associated with a retarded phase replica; a loop filter 58 for averaging the phase error of the correlation detection; a multiplier 111 for performing correlation detection using a replica in phase with the spreading code included in the received signal; an integral-dump circuit 112 for integrating the output signal of the multiplier 111 over M chip intervals; an automatic frequency control circuit 213 for detecting the carrier frequency error from the output signal of the integral-dump circuit to compensate the carrier frequency error; a demodulator 113 for making decision of the received data after compensating the received phase error of the signal obtained by the correlation detection; a multiplier 114 for performing inverse modulation of the decided data using the phase error signal outputted from the loop filter; a voltage controlled clock generator 29 for controlling the clock phase by the phase error signal outputted from the multiplier 114; and the spreading code replica generator 30 driven by the clock signal outputted from the voltage controlled clock generator 29. The modulated signal, which is down-converted by the fixed oscillation frequency substantially equal to the center frequency of the modulated signal, is deprived of harmonic components, and is multiplied by the replica of the spreading code in phase with the spreading code in the modulated signal, followed by a predetermined length of time integral. Thus, correlation peaks are detected. The correlation detection signal undergoes decision by the demodulation circuit which performs coherent detection or delay detection. On the other hand, the modulated signal is multiplied by the replica of the spreading code with an advanced phase Δ with respect to the spreading code in the modulated signal, and also multiplied by the replica of the spreading code with a retarded phase Δ, thereby eliminating the harmonic components. The error signals between the correlation associated with the advanced phase replica of the spreading code and the correlation associated with the retarded phase replica of the spreading code are added in the opposite phase by the adder 57, and its output is integrated and averaged by the loop filter 58. This results in an error voltage corresponding to the phase error between the spreading code in the received signal and the replica of the spreading code. Inverse modulation by multiplying the error voltage by the decided data outputted from the demodulator eliminates the error in the phase error signal due to the modulated signal. A delay is inserted after the phase error detection loop in order to match the absolute times of the processings of the phase error detection loop and the data decision loop. In the conventional DLL, a despread signal includes, a phase error signal component that is a component of an error between the center frequency of the received signal and the frequency of the local oscillator of the quadrature detector. The despread signal also includes data modulation components and vestigial components of a carrier signal component. To eliminate the carrier frequency offset component and the data modulation components, the despread signal may be squared by a square-law detector. This, however, will increase noise components because they are also squared, and the noise components will be added to the chip phase error, thereby increasing phase jitter. Accordingly, it is necessary to obviate the square-law detector to prevent the noise component from increasing. In view of this, the present embodiment eliminates the carrier frequency offset component from the despread signal by the AFC, and removes the data modulation component by inversely modulating the demodulated and detected signal into the despread signal. Since the quasi-coherent detected signal includes a carrier offset signal, the detected signal is involved with phase rotations. Consequently, the carrier offset signal must be removed. This is achieved by detecting, by the automatic frequency control circuit 213, the offset component of the carrier signal from the correlation peaks in the data decision loop, and by correcting the two correlation detected signals of the chip phase error detection loop by the carrier offset signal in the opposite phase directions. SUPPLEMENTS Fig. 8 is a block diagram showing detailed hardware of the correlation detector described in the second embodiment. In this figure, the same elements are designated by the same reference numerals as in Fig. 7. In Fig. 8, the reference numerals 304 designates a 90-degree phase shifter, 65 and 66 designate A/D converters, 308 and 309 designate complex multipliers, 313 designates a delay circuit, 314 designates a complex multiplier, and 317 designates a decision circuit. The reference numeral 410 designates a carrier frequency error compensation circuit, and 416 designates an automatic frequency control circuit. In the correlation detector, a received IF modulated signal is quadrature-detected by the quadrature detector. The quadrature-detected I and Q channel signals are deprived of harmonic components, and are converted into digital values by the A/D converters 65 and 66, followed by the correlation detection by applying complex signal processings on I (In-phase) and Q (Quadrature) components. The correlation detection is carried out by complex multiplication of the modulation spread signal by the I and Q components of the replicas of the spreading code. The two replicas of the spreading code is the same if the in-phase and quadrature components of the primary modulated signal are spread by the same spreading code. The operation will now be described when the primary modulation is QPSK and the secondary modulation is BPSK. The data to be modulated are primary modulated independently by binary data which are independently set for I and Q channels (QPSK modulation). The I and Q channel data are spread (secondary modulated) by the same spreading codes. The input signal to the receiver is expressed by the following equation. The signals after multiplying the input signal of equation (3) by the replicas of the spreading code at the receiver will be expressed as follows if a conventional calculation method is applied. The chip phase error signal at the output of the loop filter is expressed by the following equation. As shown by equation (18), since modulated signal powers of individual symbol components are multiplied by the phase error signal in the primary QPSK modulation, the primary modulated signal components can be eliminated by inversely modulating the I and Q components of the data after decision into the phase error signal. INDUSTRIAL APPLICABILITY As described above in detail, according to the present invention, the correlation detection during the acquisition phase which requires a high speed synchronization is carried out in the initial acquisition circuit using a matched filter, and the correlation detection during the tracking phase which requires power saving rather than high speed synchronization is performed by the sliding correlation detector. This makes it possible to achieve the high speed acquisition, and power saving during the tracking because the power consumption of the initial acquisition circuit during the tracking is negligible. In addition, according to the present invention, since the tracking loop of the received chip phase eliminates the primary modulated signal components which are included in the phase error signal of the replica signals, components only depending on the crosscorrelation can be extracted. This makes high accuracy tracking possible.

Claims: A synchronising apparatus including a correlation detector (44) for maintaining synchronisation between a received signal and a despreading code by using a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said correlation detector (44) comprising: a spreading code replica generating means (30) for generating a phase advanced replica of a CDMA spreading code with an advance phase and a phase retarded replica of the CDMA spreading code with a retarded phase with respect to the received signal; first multiplying means (47) for multiplying said received signal by said phase advanced replica of the CDMA spreading code so as to output a first correlation detection signal; and second multiplying means (48) for multiplying said received signal by said phase retarded replica of the CDMA spreading code so as to output a second correlation detection signal; the apparatus being characterised in that it comprises: carrier frequency error compensating means (208) for compensating for a carrier frequency error associated with said first and second correlation detection signals; and adding means (57) for summing in opposite phase said first and second correlation detection signals, which have been compensated by said carrier frequency error compensating means (208); an averaging circuit (58) for averaging an output signal of said adding means along a time axis; and wherein said spreading code replica generating means (30) is further operable to generate an in phase replica of said CDMA spreading code with the same phase, and wherein the correlation detector further comprises: third multiplying means (111) for multiplying said received signal by said in-phase replica of the CDMA spreading code from said spreading code replica generating means (30); integrating circuit means (112) for integrating an output signal of said third multiplier means; automatic frequency control circuit means (213) for detecting the carrier frequency error based on an output of said integrating circuit means (112), to output it to said carrier frequency error compensator (208); a demodulator (113) for generating a decided signal of received data based on an output signal of said automatic frequency control circuit means (213); an inverse modulation circuit (114) for inversely modulating an output signal of said averaging circuit (58) by the decided signal of the received data; and clock generating circuits (115, 29) for generating a clock signal whose phase is controlled by an output from said inverse modulation circuit (114) to control said spreading code replica generator (30). A synchronising apparatus in accordance with claim 1, for establishing synchronisation between a received signal and a despreading code by using a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said synchronising apparatus comprising: a switching circuit (42) operable to receive the received signal; an initial acquisition circuit means (43) for receiving an output of said switching circuit and for taking a correlation between said output and a reference spreading code so as to output a correlation value; an acquisition decision circuit means (45) connected to said initial acquisition circuit (43), for judging completion of synchronisation based on the correlation value and for outputting the judgement result to said switching circuit means (42); and wherein said correlation detector (44) is operable to receive another output of said switching circuit means and to take a correlation between said other output and a reference spreading code generated in a spreading code replica generator incorporated therein; and said switching circuit means (42) being operable to output a received signal to said initial acquisition circuit means (43) at the time of asynchronism while outputting a received signal to said correlation detector (44) after the completion of synchronisation based on the output of said acquisition detection circuit means (45). A CDMA communication apparatus provided with a synchronising apparatus in accordance with claim 1, said CDMA communication apparatus comprising: an orthogonal detector (62) for orthogonally detecting the CDMA spread received signal based on two detected signals (I, Q) whose phases are orthogonal to each other; two A/D converters (65, 66) for converting the two detected signals into digital signals, respectively, and wherein, in said correlation detector (44): said spreading code replica generating means comprises a spreading code replica generator (30) for respectively generating an in-phase replica of the CDMA spreading code with an advance phase and a phase retarded replica of the CDMA spreading code with a retarded phase with respect to said two digital signals; said first multiplying means comprises a first multiplier (47) for respectively multiplying said two digital signals by said phase advance replica of the CDMA spreading code so as to output two first correlation detection signals; said second multiplying means comprises a second multiplier (48) for respectively multiplying said two digital signals by said phase retarded replica of the CDMA spreading code so as to output two second correlation detection signals; said carrier frequency error compensating means comprises a carrier frequency error compensator (208) for compensating for respective carrier frequency errors associated with said two first and said two second correlation detection signals; said third multiplying means comprises two third multipliers (111) for respectively multiplying said two digital signals by said in-phase replica of the CDMA spreading code from said spreading code replica generator; said integrating circuit means comprises two integrating circuits (112) for respectively integrating output signals of said third multipliers; and said automatic frequency control circuit means comprises an automatic frequency control circuit (213) for detecting the carrier frequency errors based on output of said two integrating circuits, to output them to said carrier frequency error compensator. A CDMA communication apparatus provided with a synchronising apparatus for establishing synchronisation between a received signal and a despreading code by using a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said CDMA communication apparatus comprising: an orthogonal detector (62) for orthogonally detecting the CDMA spread received signal based on two detected signals (I, Q) whose phases are orthogonal to each other; two A/D converters (65, 66) for converting the two detected signals into digital signals, respectively; two switching circuits (42I, 42Q) for inputting the two digital signals, respectively; two initial acquisition circuits (43I, 43Q) for inputting one of outputs of said two switching circuits and taking a correlation between the output and a reference spreading code so as to output a correlation value; acquisition decision circuits (45) respectively connected to said two initial acquisition circuits, for judging completion of synchronisation based on the correlation value and for outputting the judgement result to said switching circuit; and synchronising apparatus including a correlation detector (44) in accordance with claim 1, said correlation detector being operable to input the other output of said two switching circuits and taking a correlation between the output and a reference spreading code generated in a spreading code replica generator incorporated therein; said two switching circuits (42I, 42Q) being operable to output a received signal to said initial acquisition circuit at the time of asynchronism while outputting a received signal to said correlation detector after the completion of synchronisation based on the output of said acquisition decision circuit; and wherein, in said correlation detector (44), said spreading code replica generating means (30) is operable to generate said in-phase replica, said phase advanced replica, and said phase retarded replica with respect to said two digital signals; said first multiplying means comprises two first multipliers (47) for respectively multiplying said two digital signals by said phase advanced replica of the CDMA spreading code so as to output first correlation detection signals; said second multiplying means comprises two second multipliers (48) for respectively multiplying said two digital signals by said phase retarded replica of the CDMA spreading code so as to output second correlation detection signals; said carrier frequency error compensating means comprises a compensator (208) operable to compensate for respective carrier frequency errors associated with said two first and said two second correlation detection signals; said third multiplying means comprises two third multipliers (111) for respectively multiplying said two digital signals by said in-phase replica of the CDMA spreading code from said spreading code replica generator; said integrating circuit means comprises two integrating circuits (112) for respectively integrating output signals of said third multipliers; and said automatic frequency control circuit means comprises an automatic frequency control circuit (213) for detecting the carrier frequency errors based on outputs of said two integrating circuits, to output them to said carrier frequency error compensator. A synchronising method using a correlation detector for tracking between a received signal and a despreading code by the use of a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said method comprising the steps of: generating an in-phase replica of a CDMA spreading code with the same phase, a phase advanced replica of the CDMA spreading code with an advance phase and a phase retarded replica of the CDMA spreading code with a retarded phase with respect to the received signal; firstly multiplying said received signal by said phase advanced replica of the CDMA spreading code so as to output a first correlation detection signal; secondly multiplying said received signal by said phase retarded replica of the CDMA spreading code so as to output a second correlation detection signal; compensating for a carrier frequency error associated with said first and second correlation detection signals; summing in opposite phase said first and second correlation detection signals, which have been compensated at said carrier frequency error compensating step; averaging an output at said summing step along a time axis; third multiplying said received signal by said in-phase replica of the CDMA spreading code; integrating an output at said third multiplying step; detecting the carrier frequency error based on an output at said integrating step so as to use it at said carrier frequency error compensating step; generating a decided signal of received data based on an output at said integrating step; inversely modulating an output at said averaging step by the decided signal of the received data; and controlling a phase of a replica code to be output based on the output at said inversely modulating step. A synchronising method for establishing synchronisation between a received signal and a despreading code by using a correlation between the received signal and the despreading code, the received signal being CDMA spread and the despreading code being used for despreading the received signal, said synchronising method comprising the steps of: taking a correlation between the received signal and a reference spreading code so as to perform initial acquisition; and taking a correlation between the received signal and the reference spreading code for tracking, after completion of the synchronisation, in accordance with the method of claim 5.

Inventors: ADACHI FUMIYUKI, DOHI TOMOHIRO, SAWAHASHI MAMORU, ADACHI, FUMIYUKI, DOHI,TOMOHIRO, SAWAHASHI, MAMORU, ADACHI, FUMIYUKI, 2-35-13,TAKAFUNEDAI KANAZAWA-KU, DOHI,TOMOHIRO, TOMIOKA-DAIICHI-DOKUSHINRYO A-408, SAWAHASHI, MAMORU, 5-42-188,URAGA-CHO
Applicants: NTT DOCOMO INC, NTT DOCOMO, INC.
Application year: 1994
Main classification: H04L7
IPC codes: H04B7, H04B1